1. Technical Field
The present disclosure relates to electrosurgical instruments used for open and endoscopic surgical procedures. More particularly, the present disclosure relates to a method of manufacturing tissue sealing electrodes for sealing vessels and vascular tissue.
2. Description of Related Art
Monopolar and bipolar instruments are among the most commonly utilized instruments in the field of electrosurgery. Briefly, monopolar instruments utilize one or more active electrode(s) that are associated with a clamping electrode (e.g., jaw members) and a remote patient return electrode or pad that is attached externally to the patient. Bipolar electrosurgical forceps utilize two generally opposing electrodes. Both electrodes are generally disposed on an inner facing or opposing surfaces of the jaw members which are, in turn, electrically coupled to an electrosurgical generator.
Essentially, during monopolar surgical treatment, energy travels from the active electrode(s) to the surgical site, through the patient and to the return electrode or pad. In the situation where more than one electrode is utilized, all of the active electrodes are charged to the same electric potential. On the other hand, during bipolar surgical treatment, each opposing electrode is charged to a different electric potential. Since tissue is a conductor of electrical energy, when the electrodes are utilized to clamp or grasp tissue therebetween, the electrical energy can be selectively transferred from one electrode to the other electrode, through the tissue, to effectively seal the tissue.
The construction and mechanics of surgical electrodes both play a major role in affecting a proper seal with tissue and vessels, especially larger vessels. For example, the seal quality may be affected by the pressure applied to the vessels and/or the sealing area of the electrodes. Accordingly, the rate and effectiveness at which tissue and/or vessels are sealed depends on the jaw pressure and the sealing area (e.g., surface area of electrode) of the jaw members. With this concept in mind, a larger jaw requires more energy to compensate for the greater surface area, alongside with the greater amount of tissue being clamped or grasped by the larger jaw members. However, in larger jaw members, when the pressure is sufficiently increased, less electrosurgical energy is required. This is evidenced by the bioheat equation and the Arrhenius function, which confirms that temperature is related to surface area and tissue heating becomes a function of temperature with respect to time, as shown below in the bioheat equation (1):
                    T        =                                            1                              σρ                ⁢                                                                  ⁢                c                                      *                          J              2                        ⁢            t                    +                      T            0                                              (        1        )            
where “T” is temperature, “σ” is Stefan-Boltzmann constant, “ρ” is density of tissue, “c” is the specific heat of tissue, “J” is the current density and “t” is time. It is important to note that current density depends on the area through which current is conducted. For example, a small area can amplify the effect of current on temperature.
As discussed above, tissue heat is calculated using the Arrhenius function (2) shown below:
                    Ω        =                  ∫                      A            ⁢                                                  ⁢                          ⅇ                              (                                                                                                    -                        E                                            ⁢                                                                                          ⁢                      a                                        R                                    *                                      1                    T                                                  )                                      ⁢                          ⅆ              t                                                          (        2        )            
where “Ω” is a dimensionless burn parameter (e.g., Ω=1 means first degree burn), “A” is a frequency factor, “E” is the activation energy, “R” is the universal gas constant, and “T” is tissue temperature.
With reference to the equations (1) and (2), the sealing quality of tissue (with respect to current density delivered over time) and the temperature of tissue that is reached for a quality seal (or even coagulation), both depend on the characteristics of the tissue clamped, held and/or grasped between the jaw members (e.g., 1/σρc). However, it should be noted that calculations of equations (1) and (2) will assume that the pressure exerted on the tissue is evenly distributed and that during the clamping process the tissue flow is laminar. That is, the tissue does not form any or have any bubbles and/or gaps, while the tissue is grasped between the jaw members.
Taking the above-described phenomena into consideration, the pressure applied by the electrodes, via the jaw members, plays an important role towards the changing tissue impedance during a tissue sealing procedure that ultimately results in a successful tissue seal. Some tissue factors that correlate with the amount of pressure applied are, for example, but not limited to: the amount of volume of tissue; the density of tissue; the viscosity of the tissue; and the specific heat of the tissue. More specifically, the amount or volume of tissue grasped by the jaw members (e.g., the electrodes) determines the amount of tissue contact the electrodes require and the distance between the electrodes. With regard to tissue density, the density of tissue grasped relates to pressure because if the jaws exert a very high pressure, the stress limit of the tissue may be exceeded, thus bursting and/or rupturing a majority of cells of the tissue. Knowing or measuring the overall viscosity of the tissue is important during clamping of the jaws because discontinuities may cause turbulent flow that would cause unwanted bubbles or gaps. Specific heat of the tissue relates to: the pressure applied during the application of pressure on tissue; the changes in cell shape; and/or rupturing may affect the specific heat in some way.
Another issue that may arise during electrosurgical surgery is “arcing” between electrodes. Arcing, which is also commonly referred to as electrical arcing, is an electrical breakdown of a gas which produces an ongoing plasma discharge that results from a current flowing through nonconductive media, for example, air. Some factors that affect arcing are the so-called “clearance distance” and the so-called “creepage distance” between the electrodes. The clearance distance is the shortest distance between two conductive media measured through air. During surgery, air clearance is of concern as high transient voltages can arc over or breach a dielectric barrier. The creepage distance is the shortest path between two conductive media measured along the surface of nonconductive media. Given a high enough potential applied between two points on nonconductive media, the right environmental conditions, and sufficient time, the surface of the nonconductive media may break down resulting in an arc between conductive surfaces. This is called “tracking.” Tracking occurs only with surfaces and is not typically associated across air.